The hypotheses tested in these in vitro studies were: a) continuous wave Doppler CWD provides accurate assessment of pressure differences across prosthetic valves as a function of flow, b) CWD and phonocardiography are accurate methods for noninvasive in vitro evaluation of the hydrodynamic performance or normal and explanted prosthetic heart valves and c) these techniques also detect early prosthetic valve failure and are accurate noninvasive procedures for serial evaluation of prosthetic valve function. A physiologic pulse duplicator with pressure and electromagnetic flow transducers, and IBM AT microprocessor, and IREX Meridian ultrasonic system, and an International Acoustics, Inc. phonocardiography amplifier system were used. Eighteen new mechanical, thirty-three new bioprosthetic, and twelve explanted bioprosthetic aortic prostheses were studied. Peak pressure differences were measured at 1 cm increments from 1 to 6 cm distal to the valve annulus for determination of maximal peak pressure differences. Simultaneous maximal velocities were obtained with CWD and transformed to pressure differences with the modified Bernoulli equation. Complete evaluation of the data for all valves studied is currently in progress.